Method of improving heat stability of acrylonitrile polymer fibers



United States Patent Ofiice 3,415,611 patented Dec. 10, 1968 3,415,611 METHOD F lMlRfiVING HEAT STAEELITY Gi ACRYLONETRKLE PDLYMER Andrew I. Callahan, Newport News, and Richard E. Harder and Rupert B. Hurley, Wiiiiarnsburg, Van, assignors to The Dow Chemical @ompany, Midland, Mich, a corporation of Delaware No Drawing. Filed (let. 28, W65, Ser. No. 595,449 7 Claims. (Ci. 8--l37.5

This invention relates to a method for improving the heat stability, or resistance to discoloration upon exposure to elevated temperatures, of acrylonitrile polymer fibers. It is particularly directed to improving the heat stability of acrylonitrile polymer fibers that are prepared by wet spinning techniques and which are treated in accordance with the invention while they are in a never dried or gel condition.

Acrylonitrile polymers containing at least about 80 weight percent acrylonitrile, including homopolymeric acrylonitrile, are especially well suited as fiber-forming compositions. These compositions can be fabricated into textile fibers or filaments (fibers and filaments are used interchangeably herein) having excellent physical properties for use in a variety of applications. One widely employed means for preparing these fibers is by spinning a solution of the polymer into an aqueous coagulation bath, i.e., by wet spinning. It is frequently observed that acrylonitrile polymer fibers prepared by these methods have a tendency to discolor, especially from white to yellow and further darkening to amber-like colorations when exposed to elevated temperature for a period of time. For instance, when a white fiber filament is exposed to a temperature of about 200 C. for about minutes, the fiber frequently develops an intense yellow-orange color. It can be readily appreciated that resistance to heat discoloration in such textile fibers is highly desirable in order to avoid undesirable color formation in view of the many processes and treatments to which the textile fibers, yarns, and fabrics are put that involve elevated temperatures including dyeing, drying, laundering, pressing, ironing, etc.

One means that has been employed to solve the heat discoloration of acrylonitrile polymer fibers, at least to a partially acceptable degree, has been to incorporate an optical whitening agent in a polymer prior to spinning. This means is not entirely satisfactory, however, in that it serves principally only to mask the discoloration without actually preventing it. Also, the masking effect may not be permanent or lasting to any great extent, and, additionally, the optical whitening agents may interfere with dyeing in subsequent treatments.

A method is disclosed in US. 2,648,592, to Stanton et al., which describes the use of the alkali metal salts of ethylene diamine tetraacetic acid to remove trace amounts of metal salts from acrylonitrile polymer fibers that have been wet spun from aqueous saline solutions. The Stanton et al. process involves treating a gel filament with an aqueous solution of an alkali metal salt of ethylene diamine tetraacetic acid at a pH near 8 to remove the salt in the filaments after the filament has been coagulated and stretched. Although the Stanton et a1. process appears related, at least superficially, to the present invention, it is not to be confused with this invention. Among other things, the present invention is premised on the surprising finding that the treatment of such wet spun fibers with alkali metal salts of EDTA is not effective to heat stabilize the fibers when treated at a pH near 8. In US. 3,003,993 it is reported that acrylonitrile polymer fibers can be made color stable provided the polymer is washed with a very dilute solution of ethylene diamine tetraacetic acid. It is stated that therein the solutions containing less than 1000 p.p./m. (0.1 percent) and normally less than 100 ppm. (0.01 percent) of the acid must be employed for washing finely divided polymer before color stabilization results. It is also reported therein that the treatment is ineffective for treating the polymer already dissolved in a solvent or for treating a fiber of the polymer. When the present method is employed the gel filament can be treated with up to relatively concentrated solutions of EDTA to be exceptionally stable to discoloration caused by exposure to heat.

Problems are encountered, also, when the metal salts are removed by the alkali metal salts of ethylene diamine tetraacetic acid in a process whereby the salt removal is conducted while stretching the fiber at elevated temperatures. In such a system, the fibers have usually been prestretched to a small degree before metal salt removal with the complexing agent. Such a method requires a secondary washing of the fibers, after hot stretching, to remove complexing residues prior to drying the wet gel tow. Since the secondary washing is done at relatively high speds, wrap-ups and serious loss of production occur.

It has now been discovered that these and other disadvantages are overcome by a method of improving heat stability of acrylonitrile polymer fibers which comprises immersing a wet spun gel filament of a polymer of an ethylenically unsaturated monomeric material containing at least weight percent acrylonitrile in an aqueous solution of between about 0.1 and about 20 weight percent, based on solution weight, of a compound selected from the group consisting of ethylene diamine tetraacetic acid and its alkali metal salts, e.g. tetrasodium salt, said aqueous solutions being maintained at a. pH of from about 1 to 7 and at a temperature between about 0 and 30 C prior to stretching of the Wet gel filament.

In preferred embodiments of the invention, whereby more efiicient and superior results are obtained, the aqueous solution is maintained at a pH between about 1.0 and 5.0; the aqueous solution is maintained at a tempeature between about 20 and 30 C.; and, the gel filament is immersed in the aqueous solution of the ethylene diamine tetraacetic acid or its alkali metal salts (which group of compounds will hereinafter be referred to as EDTA), then washed free of the complexing residue and subsequently stretched to orient the molecules of the gel filament just prior to irreversibly drying the gel filament; and, the gel filament is not immersed in the solution of EDTA until the filament has been washed essentially completely free of any residual polymer solvent or saline constituents.

Subsequently to the immersion of the gel filament in the EDTA solution, it may be subjected to other processing treatment such as the application of finishing agents or lubricants, or hot or cold stretching, before being irreversibly dried, i.e., the gel structure is destroyed, to assume the characteristics of a normal textile filament.

The wet spun fibers or filaments treated follwing the precepts of the invention have, to an unusual degree, markedly improved resistance to discoloration even when exposed to elevated temperatures for extended periods of time. The presence of EDTA has no ill effects on the fibers and it exerts no apparent interference with subsequent processing or utility of the fiber. Additionally, the EDTA is excellently compatible with and adherent to the acrylonitrile polymer fiber so that, for practical purposes, essentially permanent heat stable characteristics are imparted to the treated fiber. Preferably, the wet gel filaments are washed essentially free of the EDTA residue prior to further treatment.

The present invention is applicable to treating acrylonitrile polymer fibers which are fabricated from fiberforming acrylonitrile polymers that contain in the polymer molecule at least about 80 weight percent of polymerized acrylonitrile, including the treatin of homopolymeric acrylonitrile, which are wet spun in and with systems which are adapted to utilize aqueous coagulating liquids in the spinning operation, such as systems wherein ethylene carbonate, dimethylformamide, dimethylacetamide dimethylsulfoxide, butyrolactone, and the like or the various saline polyacrylonitrile-dissolving solvents are employed as spinning solution solvents for the polymer and are also present in nonpolymer dissolving quantities in the aqueous coagulating liquid used in the spin bath.

The utile, known aqueous saline solvents for the various fiber-forming acrylonitrile polymers and polyacrylonitrile include zinc chloride, the various thiocyanates such as calcium and sodium thiocyanate, lithium bromide, salt mixtures of the so-called lyotropic series, and others recognized by the art as has been disclosed, among other places, in United States Letters Patents Nos. 2,140,921, 2,425,192, 2,648,592, 2,648,593, 2,648,646 2,648,648, 2,648,649, and 2,949,435. Advantageously, aqueous zinc chloride solutions are used for the purpose.

Exemplary of some of the monomeric materials that may be employed with the acrylonitrile in the preparation of acrylonitrile polymer and copolymer fiber-forming systems treated in accordance with the practice of the present invention includes allyl alcohol, vinyl acetate, acrylamide, methacrylamide, methylacrylate, vinyl pyridine, ethylene sulfonic and its alkali metal salts, vinyl benzene acid and its salts, 2-sulfoethylmethacrylate and its salts, vinyl lactams such as vinyl caprolactams and vinyl pyrrolidone, etc. and mixtures thereof.

As indicated after acrylonitrile polymer fibers have been wet spun they are most frequently water washed or Washed with an aqueous inert solution to remove any residual polymer solvent from the freshly formed filaments, thus forming an intermediate fiber product often referred to as a gel or aquagel filament. Thoroughly washed acrylonitrile polymer aquagel fibers, incidently, are usually found to contain up to about 6 parts by Weight of water (including residual extrinsic or exterior water associated therewith) for each part by weight of dry polymer therein. More frequently, washed acrylonitrile aquagel polymer fibers are found to contain from about 3 to 4 parts by weight of water for each part by weight of polymer.

The present invention can be carried out conveniently in standard spinning trains. That is, no major alterations to a conventional fiber-forming process need be undertaken. Becaue of the extreme speed at which the fibers can be treated in practicing the present invention, the ordinary and commercially useful spinning speeds can be employed while simultaneously obtaining an excellent heat stable fiber.

The gel or aquagel filaments are treated with the solution of ethylene diamine tetraacetic acid and the alkali metal salts thereof. After the filaments have been coagulated and washed essentially and completely free of any residual polymer solvent including saline constituents. It is the general practice to pass the filaments through the solution on a continuous basis as they are spun without interrupting the spinning operation. As indicated, filaments spun by wet spinning techniques are usually necessarily stretched to impact to the resulting fiber desirable physical properties. In the present invention the gel filaments are treated with the EDTA treating solution prior to the stretching operation. Unexpectedly good heat stabilizing results are obtained when the gel filaments are immersed, prior to stretching, in an aqueous solution of the EDTA. It is desirable also to at least partially wash the gel filaments after the EDTA treatment, prior to any further treatment, i.e., stretching or treating with finishing agents.

The temperature of the EDTA treating solution is beneficially between about to 30 C. Advantageously, the temperature is maintained between about 20 to 30 C.

Treating the gel filaments in solutions at ambient temeratures provide fibers exhibiting considerably better heat stability properties, as well as eliminating problems of filament breakage and wrap-up.

It has been found that the pH of the EDTA treating solution is particularly important to effectuate acceptable heat stability in the acrylonitrile polymer fibers. A pH no higher than about 7 is most desirable, and a preferred pH is from about 1 to 5. This pH range gives a great deal of latitude in the spinning process, and even though pH is an important factor, practice of the invention has shown pH control to be a relatively simple operations. Consequently, the present invention is a vast improvement over known processes that require rigid pH control within a relatively narrow range in order to produce an acceptable acrylonitrile polymer fiber.

The concentration of the EDTA, or an alkali metal salt thereof, in the treating solution is not particularly critical. Ordinarily, at least about a 0.1 weight percent solution of the EDTA should be employed to assure satisfactory heat stable fibers. Actually, concentrations up to the saturation concentration of the EDTA in the aqueous solution can be employed but no particular advantage is observed. Beneficially, solutions of from about 0.1 to about 20 weight percent of the EDTA, based on solution weight, and preferably solutions are from about 0.1 to about 2.0 weight percent are employed.

The time for the treatment, as discussed herein, is ex ceedingly rapid, particularly when consideration is made of the excellent results that are achieved. The general practice is to pass the gel filaments through a bath of the EDTA at conventional fibers spinning speeds. Generally, about a 5 second immersion time of the filaments in the EDTA solution is adequate to obtain highly acceptable heat stable fibers. Somewhat shorter and certainly longer times can be successfully employed.

The following examples serve to further illustrate the invention, wherein, unless otherwise specified, all parts and percentages are by Weight.

Example 1 A spinning solution of about 10.5 percent of a polymer of about 91.1 percent polyacrylonitrile, 7.5 percent methylacrylate, and 1.4 percent sulfoethyl methylacrylate, dissolved in an aqueous solution containing about 60 percent ZnCl was extruded through a multiorifice spinnerette into a coagulation bath containing an aqueous about 32.5 percent ZnCl solution at about 31 C. The co'agulated gel filaments were withdrawn from the coagulation bath and water washed essentially completely free of residual ZnCl The copolymer gel filaments were washed in EDTA solutions at a pH of about 5, at ambient temperatures (24 C.) and subsequently washed in water to remove the residual EDTA solution. Samples of the copolymer gel filaments submitted for heat stability test were stretched three times in boiling water and subsequently dried at C. for four minutes. The data obtained from these tests are contained in Table I.

Example 2 A homopolymer spinning solution containing about 10 percent acrylonitrile polymer dissolved in an aqueous solution of about 60 percent ZnCl was extruded through a multiorifice spinnerette into a coagulation bath containing an aqueous about 44 percent ZnCl salt solution at 13 C. The coagulated gel filaments were then washed essentially free of salt solution, treated and tested in the same manner as in Example 1, above. The data obtained from testing the treated homopolymer gel filaments are found in Table I.

Example 3 A spinning solution of about 10.5 percent of a polymer of about 90.7 percent polyacrylonitrile, 8.0 percent methylacrylate, and 0.23 percent sulfur as sulfoethyl methacrylate was spun into filaments according to the method of Example 1, above. The resulting filaments were treated and tested as in Example 1, and the results thereof are found in Table I.

1 Heat stability determination.

The resultant fiber products from the examples were cut into small pieces and chopped into a slurry in a Waring Blendor. The slurry was then filtered through a Buchner type tunnel to form a pad which was then dried. The dried fiber pads of the samples were then placed between the plates of an AATCC Scorch Tester at 205 C. for five minutes. The measuring head of a Photovolt Corporation refiectometer fitted with a green tri stimulus filter was placed first on the unscorched portion of the fiber pad and the instrument adjusted for a reflectance reading of 100; a reflectance reading was then taken on the scorched portion of the fiber pad. The diiference in reflectance is a measure of the color formation due to heat. The recorded values are the number of units below the control value of 100. The smaller the recorded value the less change from the unscorched reading of 100 and hence the better the heat stability of the fiber. The results are set forth in Table I.

What is claimed is:

1. In a method for preparing heat stable acrylonitrile polymer fibers comprising immersing the wet spun gel filament of a polymer of an ethylenically unsaturated monomeric material containing at least about weight percent of acrylonitrile in an aqueous solution of between about 0.1 and about 20 weight percent based on solution weight, of a compound selected from the group consisting of ethylene 'diamine tetraacetic acid and its alkali metal salts, the improvement which comprises maintaining said aqueous solution at a pH of less than about 7.0, at a temperature between about 0 to 30 C. and said immersion being done prior to stretching of the Wet gel filament.

2. The method of claim 1, wherein the pH of said solution is between about 1.0 and 5.0.

3. The method of claim 1, wherein said solution is at a temperature of between about 20 and 30 C.

4. The method of claim 1, wherein said gel filament is stretched to orient the molecule thereof subsequent to being immersed in said solution.

5. The method of claim 1, wherein said solution is about a 0.1 to about 2.0 Weight percent solution of said compound.

6. The method of claim 1, wherein said compound is the sodium salt of ethylene diamine tetraacetic acid.

7. The method of claim 1, wherein said gel filament is spun from an aqueous saline solution that is a solvent for polyacrylonitrile.

References Cited UNITED STATES PATENTS 2,648,592 8/1953 Stanton et al 8--137.5 3,003,993 10/1961 Wooten 8-1375 XR MAYER WEINBLATT, Primary Examiner.

US. Cl. X.R.

260 296, 883; 264 182; Ill-138.8 

